Conventional technologies will be described below.
Conventionally, in an internal combustion engine (hereinafter referred to also as a supercharged engine) including a supercharger that feeds supercharged intake air into cylinders, temperature of intake air rises through its compression by the supercharger. When the temperature of intake air rises, knocking is easily generated at the time of high load of the engine in addition to deterioration of filling-up efficiency of intake air with respect to the engine. As a measure against this, there is disposed an intercooler for cooling the supercharged intake air which has been compressed and increased in temperature by the supercharger. The intercooler may be of an air-cooling type or a water-cooling type. Particularly, the water-cooling type intercooler can be disposed in an intake pipe on a downstream side of the supercharger, for example, in a surge tank room of an intake manifold (see, for example, JP-T-2010-510425).
An internal exhaust gas recirculation (EGR) system that opens an intake valve in an exhaust stroke of the engine to be capable of returning a part of exhaust gas into an intake port is known. Internal EGR gas which is the part of exhaust gas returned into the intake port has higher temperature than external EGR gas. The internal EGR gas heats the intake valve, to which fuel injected through an injector that can inject fuel into the intake port is easily attached, and evaporation of fuel is thereby promoted. However, in an intake stroke immediately after being returned into the intake port, the high-temperature internal EGR gas is drawn into the cylinder again together with the high-temperature supercharged intake air compressed by the supercharger. Accordingly, the drawn internal EGR gas still has high temperature, and temperature in a combustion chamber of each cylinder easily rises. The high-temperature internal EGR gas flows into the combustion chamber, and knocking may be caused when the temperature in the combustion chamber becomes high.
For this reason, there is known an internal EGR system that has cylinders and includes one cooling tube and cooling jackets for cooling the internal EGR gas returned into the intake port of each cylinder by coolant (see, for example, Japanese Patent No. 4563301). The cooling tube includes intake port insertion parts which are inserted into vicinity of the intake valve, i.e., deep into the intake port. The coolant flows from an inlet-side cooling tube in a surge tank through the intake port insertion part in each intake port in this order to cool each branch pipe of the intake manifold and each intake port in sequence. Then, the coolant is delivered to the outside through an outlet-side cooling tube in the surge tank. As described above, the coolant flowing through the cooling tube flows serially from an intake port of a cylinder #1 to an intake port of a cylinder #4.
As regards the cooling jackets, the cooling jackets surrounding their corresponding branch pipes are connected to one coolant main pipe inserted into the surge tank through inlet and outlet branch pipes. In this case as well, the coolant flows from the cooling tube through the cooling jacket of the cylinder #1 and is returned to the cooling tube again. The coolant flows from a downstream side of this return part to the cooling tube through the cooling jacket of a cylinder #2 and is returned to the cooling tube again. The coolant flows from a downstream side of this return part to the cooling tube through the cooling jacket of a cylinder #3 and is returned to the cooling tube again. The coolant flows from a downstream side of this return part to the cooling tube through the cooling jacket of a cylinder #4 and is returned to the cooling tube again. In this manner, the coolant flowing through each cooling jacket flows in series from the branch pipe of the cylinder #1 toward the branch pipe of the cylinder #4.
However, in the conventional internal EGR system, both of the supercharged intake air which has flowed into the intake manifold and the internal EGR gas which has flowed into each intake port are cooled by the cooling tube and the cooling jackets. Accordingly, in the case of a four-cylinder engine, there is an issue of deficiency in cooling performance of the supercharged intake air and the internal EGR gas only by means of the cooling tube for each cylinder that is divided into four quarters and the cooling jackets.
The reason for this is that the entire flow of supercharged intake air needs to be cooled with the volume limited. Moreover, in order to efficiently cool the supercharged intake air, it is necessary to expand the volume of the intercooler that is constituted of the cooling tube and the cooling jackets. If the intercooler is extended in an intake air flow direction (direction X) in the intake manifold, a size of the intercooler in the direction X is increased. If the intercooler is extended in a height direction (direction Y) of the intake manifold that is perpendicular to the direction X, it is necessary to increase a gap between fastening points at which the intake manifold and a cylinder head are fastened together. As a result of these, due to the increase of the intercooler in size in the direction X and direction Y, the intake manifold itself grows in size so that installability of the intake manifold in an engine compartment of a vehicle deteriorates.
Furthermore, in the conventional internal EGR system, the coolant flowing through the cooling tube flows serially from the intake port of the cylinder #1 to the intake port of the cylinder #4. For this reason, a distance from a coolant inlet through which the coolant flows into the cooling tube to a coolant outlet through which the coolant flows out of the cooling tube is great in a longitudinal direction (branch pipe arranging direction: direction Z) of the surge tank room. Consequently, the temperature of coolant increases for each intake port in stages through its heat exchange with the internal EGR gas. As a result, a difference is made in cooling performance of the internal EGR gas between the cylinders of the engine.